Transducers Outline PDF

Transducers Outline 1 / 58 Classification of Transducers Transducers can be classified 1. on the basis of transduction form used 2. as primary and secondary transducers 3. as passive and active transducer 4. as transducers and inverse transducers...

Transducers Outline 1 / 58 Classification of Transducers Transducers can be classified 1. on the basis of transduction form used 2. as primary and secondary transducers 3. as passive and active transducer 4. as transducers and inverse transducers 1 / 57 Electrical Param- Principle of Operation Applications eter and Class Resistance Potentiometer Resistance variation by Pressure, dis- an external force on placement slider Strain gauge Resistance variation of Force, torque, dis- wire or semiconductor placement by elongation or com- pression Pirani gauge or Resistance variation of Gas flow, gas hot wire meter hot wire with temper- pressure ature difference due to stream of gas 2 / 57 Electrical Param- Principle of Operation Applications eter and Class Resistance ther- Resistance variation of Temperature, ra- mometer pure metal wire with diant heat large positive tempera- ture coefficients Thermistor Resistance of certain Temperature, metal oxides with nega- flow tive temperature coeffi- cients Resistance hyge- Resistance of conduc- Relative humidity ometer tive strip changes with moisture content Photoconductive Resistance variation of Photosensitive re- cell cell with light lay 3 / 57 Electrical Param- Principle of Operation Applications eter and Class Capacitance Variable capac- Distance between plates Displacement, itance pressure is varied pressure gauge Capacitor micro- Sound pressure on mov- Speech, music, phone able diaphragm varies noise capacitance Dielectric gauge Variation in capacitance Liquid level, by changes in the dielec- thickness tric constant 4 / 57 Electrical Param- Principle of Operation Applications eter and Class Inductance Magnetic circuit Self-inductance or mu- Pressure, dis- transducer tual inductance is var- placement ied by changes in mag- netic circuit Reluctance pick Reluctance variation by Pressure, dis- up change in the position of placement, vibra- iron core tions, position Differential trans- Differential voltage of Pressure, force, former two secondary windings displacement, of a transformer is var- position ied by position of mag- netic core Eddy current Inductance variation Displacement, gauge with proximity of eddy thickness current plate Magnetostriction Magnetic properties are Force, pressure, gauge varied by pressure and sound 5 / 57 Electrical Param- Principle of Operation Applications eter and Class Voltage and current (Pas- sive) Hall effect pickup Voltage across s semi- Magnetic flux, conductor plate when current, power magnetic flux interacts with an applied current Ionization cham- Electron flow induced Particle counting, ber by ionization of gas due radiation to radio-active radia- tion Photoemissive Electron emission due Light, radiation cell to incident radiation Photomultiplier Secondary electron Light, radiation, tube emission due to in- photosensitive re- cident radiation on lays photoemissive cathode 6 / 57 Electrical Param- Principle of Operation Applications eter and Class Voltage and current (Active) Thermocouple Voltage across the junc- Temperature, and thermopile tion of two dissimilar heat flow, radia- metals or semiconduc- tion tor when that junction is heated Moving coil gen- Motion of a coil in Velocity, vibra- erator magnetic field generates tions voltage Piezoelectric Voltage due to external sound, vibrations, pickup force on crystalline ma- acceleration, terials like quartz pressure changes Photovoltaic cell Voltage across semicon- Light meter, solar ductor junction when cell radiant energy stimu- lates the cell 7 / 57 Resistive Transducers ▶ Resistive Transducers are preferred to those employing other principles. ▶ This is due to their suitability for both a.c and d.c ρL ▶ Resistance of a metal conductor: R = A ▶ Resistive transducers can use variation one of the parameters ▶ Translational and rotational potentiometers ▶ Translational displacement ▶ Rotational displacement ▶ Strain gauges: Change of resistance when strained ▶ Measurement of displacement, force and pressure ▶ Resistance thermometers and Thermisotrs ▶ Resistivity of materials change with temperature ▶ Used to measure temperature 8 / 57 Potentiometers ▶ It is also called as POT, has a sliding contact called wiper ▶ Pots can be ▶ Linear pots: Translational ▶ Rotary pots: Rotational ▶ Helipots: Translational and rotational ▶ Translational pots have a stroke of 2mm to 0.5m ▶ Rotational pots can have full scale angular displacement as small as 10◦ and can go upto 357◦. ▶ Helipots can measure upto 3500◦ R (x /x ) x θ ▶ e0 = p i t ei = i ei ; ei = i ei Rp xt θt ▶ Loading effect due to meter resistance Rm : xi ▶ Let K = xt e0 K = ei K(1 − K)(Rp /Rm ) + 1 9 / 57 Strain Gauge ▶ Strain Gauge: Change in resistance due to change in dimension of elastic material under stress s. ρL R= A By differentiating dR ρ ∂L ρL ∂A L ∂ρ = − 2 + ds A ∂s A ∂s A ∂s Divide with R on both sides 1 dR 1 ∂L 1 ∂A 1 ∂ρ = − + R ds L ∂s A ∂s ρ ∂s Using the relation between cross-section area and diameter π 2 ∂A πD ∂D 1 ∂A 2 ∂D A= D ; = ; = 4 ∂s 2 ∂s A ∂s D ∂s 10 / 57  1 dR 1 ∂L 2 ∂D 1 ∂ρ = − + R ds L ∂s D ∂s ρ ∂s lateral strain ∂D/D Poisson’s ratio ν = =− longitudinal strain ∂L/L 1 dR 1 ∂L 2 ∂L 1 ∂ρ = +ν + R ds L ∂s L ∂s ρ ∂s For small variations, ∆R ∆L ∆L ∆ρ = + 2ν + R L L ρ ∆R/R ∆ρ/ρ Gauge factor Gf = = 1 + 2ν + ∆L/L ∆L/L 11 / 57 Resistance Transducers ▶ Resistance Thermometers: Resistance of a conductor changes with temperature ▶ R = R0 (1 + α1 T + α2 T 2 +... + αn T n +....) ▶ Resistance temperature detectors (RTDs) 1. Change in resistance with temperature should be as large as possible 2. Should have high resistivity, so that minimum volume is sufficient 3. Should have continuous and stable relationship between resistance and temperature ▶ Thermistors: Are generally composed of semiconductor materials ▶ Most thermistors have negative temperature coefficients 12 / 57 Inductance Transducers ▶ Generally, one of the following principles is used 1. Change of self inductance 2. Change of mutual inductance 3. Production of eddy currents ▶ Transducers based on change of self inductance: ▶ A coil with N number of turns and reluctance R has a self N2 inductance L = R ▶ A coil with length l and cross-section area A has a l reluctance R = µA N 2 µA L= = N 2 µG; where G is geometric form factor l ▶ Variation of inductance may be caused by 1. Change in number of turns, N 2. Change in geometric configurations, G 3. Change in permeability, µ 13 / 57 ▶ Differential Output: Sensitivity and accuracy will be much higher in measuring ∆L when compared to L + ∆L ▶ Transducer can be designed to produce both increase and decrease of self inductance ▶ Difference of these outputs give 2∆L ▶ Advantages of differential output are are: 1. Higher sensitivity and accuracy 2. Output is less affected by external magnetic fields 3. Effective variations due to temperature changes are reduced 4. Effects of changes in supply voltage and frequency are reduced ▶ Change of Mutual Inductance: Transducers working on this principle use multiple coils √ ▶ Mutual inductance between two coils is M = K L1 L2 ; L1 and L2 are self-inductances of two coils, K is coefficient of coupling ▶ Mutual inductance can be converted to self-inductance by series connection, with variation of L1 + L2 − 2M to L1 + L2 + 2M 14 / 57 ▶ Self inductance of each coil is constant, only mutual inductance is variable based on relative displacement ▶ It can be translational or rotational ▶ In differential arrangement, fixed coil is divided into two parts ▶ Movement of coil increases mutual inductance in part by ∆M and decreases it in other part by ∆M ▶ Air cored and iron cored coils ▶ Air Cored Coils ▶ Can be operated at higher frequencies due to absence of eddy current losses ▶ Inductance is independent of current in the coil as permeability of air is constant ▶ Iron cored coils ▶ Smaller size due to high permeability of iron cores ▶ Less likely to cause and get affected by external magnetic fields ▶ Inductance is not constant and depends on current ▶ Higher eddy current losses at higher frequencies ▶ Maximum value of 20kHz and 2kHz for accurate measurements 15 / 57 ▶ Production of Eddy currents in conducting plate near to a coil carrying a.c ▶ The plate acts as short-circuited secondary winding ▶ The eddy currents produce a magnetic field acting against the field produced by coil ▶ This reduces the flux and inductance of coil ▶ Nearer the plate, higher are the eddy currents ▶ Variation of inductance is a function of distance ▶ There can be various arrangements 1. The plate can be at right angle to axis of coil 2. A conductive sleeve can be run over the coil coaxially 16 / 57 Linear Variable Differential Transformer ▶ It has a primary coil, two secondary coils and a core ▶ Position of core determines the voltage induced in secondary coils ▶ Output is difference of voltages in secondary coils ▶ At null position both the secondary coils have same voltage ▶ Displacement is measured by magnitude of differential voltage ▶ Direction is determined by phase of output voltage ▶ On one side, the phase is same as input ▶ On the other side, it is 180◦ out of phase ▶ A controller can be used to restore the core to NULL position ▶ The output is linear upto small displacements (around 5mm), after that it deviates from straight line ▶ Ideally, output at NULL position should be zero ▶ Due to harmonics, imbalances in magnetic and / or electrical, and temperature effects, there may be residual voltage. Improved technology reduces residual voltage. 17 / 57 Advantages of LVDT ▶ High range: Can measure 1.25mm to 250mm ▶ With 0.25% full scale linearity, it can measure as small as 0.003mm ▶ Dynamic response slower than 2.5kHz excitation signal ▶ Friction and Electrical Isolation: No wear and tear due absence of physical contact between core and coils ▶ Immunity to external effects: Isolation from corrosive fluids, coils can be sealed with non-magnetic material. ▶ High output and high sensitivity: Around 40V/mm ▶ Ruggedness: Can tolerate high degree of shock ▶ Low Hysteresis: Repeatability is excellent ▶ Low power consumption: Less than 1W 18 / 57 Disadvantages of LVDT ▶ Relative large displacement required for appreciable output ▶ Sensitive to stray magnetic fields, but shielding is possible ▶ Performance affected by vibrations ▶ Receiving instrument should work with a.c ▶ Dynamic response limited by mass of core and frequency of input voltage ▶ Temperature can affect the performance, it can case phase shifts ▶ Manganin wire can be used to minimize the temperature effects, but sensitivity of manganin is 1/5 of that of copper wire 19 / 57 ▶ Applications of LVDT: ▶ To measure displacements ranging from fraction of a mm to a few cm ▶ Can be used as a primary transducer to measure displacement ▶ Can be used as a secondary transducer to measure force, weight, pressure, etc. ▶ Examples 1. Weight or pressure exerted by liquid in a tank with the help of load cells 2. Thickness of a metal sheet 3. Tension in a string 4. Complex measurements with multiple LVDTs ▶ Rotary Variable Differential Transformer (RVDT) ▶ It is a variation of LVDT to measure rotation ▶ The core is cam shaped and symmetrical ▶ Differential voltage of secondary coils is zero at NULL ▶ With magnitude and phase information, magnitude and direction of rotation can be obtained 20 / 57 Capacitive Transducers ▶ Capacitance of a parallel plate capacitor is, ϵA ϵr ϵ0 A C= = d d A = Effective area between the plates d = Distance between the plates ϵ = ϵr ϵ0 = Permittivity of the medium ϵr = Relative permittivity ϵ0 = Permittivity of free space ▶ Change of capacitance can be caused by 1. Changing the effective area of overlapping, A 2. Changing the distance d between the plates 3. Change in the dielectric constant ▶ Change of physical parameters can be caused by displacement, force, pressure etc. ▶ Change of capacitance can be measured by bridge circuits ▶ Generally, output impedance of capacitive transducer is high ▶ To measure displacement, capacitive transducers use ▶ Change in overlapping area ▶ Change in distance between plates 21 / 57 Capacitance of Parallel Plate and Cylindrical Capacitors ▶ Parallel Plate Capacitor ▶ Electric field between parallel plates of overlapping length σ Q x and width w E = ; σ = charge density = ϵ A Fd ▶ Potential difference V = = Ed Q Q Q Qϵ ϵA ϵxw ▶ C= = = = = V Ed σd d d ▶ Cylindrical Capacitor ▶ Electric field between cylinders of overlapping length x and diameters D1 , D2 and D1 < D2 is given by, λ Q E= ; λ = charge per unit lenght = 2πrϵ x ▶ Voltage between the coaxial cylinders is given by Z D2   λ 1 λ D2 V = dr = ln 2πϵ D1 r 2πϵ D1 Q 2πϵx ▶ Capacitance C = =   V ln D2 D1 22 / 57  ∂C ▶ Sensitivity of a capacitor is given by S = ∂x w ▶ Parallel plate capacitor: S = ϵ d 2πϵ ▶ Cylindrical capacitor: S =   ln DD1 2 ▶ Sensitivity is constant, the relationship between capacitance and displacement is linear ▶ Capacitive pickup can be used to measure rotation ▶ Two semi-circular plates of radii r are used θr2 ▶ Area between plates is given by A = 2 ϵθr2 ▶ Capacitance C = 2d ∂C ϵr2 ▶ Sensitivity S = = ∂θ 2d ▶ Sensitivity is constant, the relationship between capacitance and rotation is linear 23 / 57 Capacitance pickup with distance ▶ Capacitance is inversely proportional to distance between ϵA plates C = d ∂C ϵA ▶ Sensitivity S = =− 2 ∂d d ▶ C vs d is a nonlinear relationship and hyperbolic in nature ▶ It is linear over small range ▶ It can be made approximately liner by using a material with high dielectric constant like mica ▶ Sensitivity can be increased with reduction in distance ▶ Distance between plates is limited by breakdown voltage, 3kV/mm ▶ Applications with displacement 1. It can have cantilever arrangement loaded with spring 2. Silvered quartz diaphragms can be used in pressure gauges ▶ Stator and rotor arrangement can be used to measure rotation 24 / 57 Differential Arrangement ▶ It uses a movable plate M between two fixed plates P1 , P2 ϵA ϵA ▶ C1 = and C2 = are capacitances w.r.t movable plate d d ▶ When a.c voltage E is applied across fixed plates EC2 EC1 ▶ Voltages across C1 , C2 are E1 = , E2 = C1 + C2 C1 + C2 ▶ At NULL position, ∆E = E1 − E2 = 0 ▶ Let M be moved towards P1 by an amount of x ϵA ϵA ▶ C1 = and C2 = d−x d+x ϵA E d+x E(d − x) ▶ E1 = = ϵA ϵA 2d d−x + d+x ϵA E d−x E(d + x) ▶ E2 = = ϵA ϵA 2d d−x + d+x Ex ▶ Differential voltage ∆E = E2 − E1 = d ∆E E ▶ Sensitivity S = = x d ▶ Used to measure 10−8 mm to 10mm 25 / 57 Variation of Dielectric Constant ▶ A linear displacement of dielectric material changes the overall dielectric constant ϵ wl ϵ ϵ wl ϵ w ▶ C = 0 1 + 0 r 2 = 0 (l1 + l2 ϵr ) d d d ▶ When the dielectric material is moved to reduce air volume e = ϵ0 w (l1 − x + (l2 + x)ϵr ) = ϵ0 w (l1 + l2 ϵr ) + ϵ0 wx (ϵr − 1) ▶ C d d d ϵ wx ▶ Change in capacitance ∆C = 0 (ϵr − 1) d ∆C ϵ0 w ▶ Sensitivity S = = (ϵr − 1) is a constant x d ▶ Relationship between change in capacitance and displacement is linear ▶ Measurement of liquid level ▶ With two concentric cylinders in a tank with non-conducting liquid ϵ h + ϵ2 h2 ϵ1 h1 + ϵ2 h2 ▶ C = 2πϵ0 1 1   = 2πϵ0   ; r2 = r1 + a r2 ln r1 ln 1 + ra1 26 / 57 Piezo-electric Transducers ▶ Certain materials have the property to generate electric potential across its surfaces when deformed under stress. ▶ It is direction sensitive. ▶ We call them as piezo-electric or electro-resistive elements ▶ The effect is reversible, it deforms when potential is applied ▶ Rochelle salt (KN aC4 H4 O6 4H2O),ammonium dihydrogen phosphate (N H4 H2 P O4 ), lithium sulphate (Li2 SO4 ), dipotassium tartarate (C2 H4 K2 O6 ), potassium dihydrogen phosphate (KH2 P O4 ), quartz and ceramics A and B ▶ Except quartz and ceramics A and B, rest are man-made. ▶ The effect can be observed with 1. Thickness expansion 2. Transverse expansion 3. Thickness shear 4. Face shear 5. Twisting and/or bending 27 / 57 ▶ The material acts as a combination of charge generator and Q capacitor V = C ▶ Charge Q produced by force F is given by, Q = D × F D is charge sensitivity of material stress F/A ▶ Young’s modulus E = = strain ∆t/t AE∆t ▶ F = t DAE∆t Q= t Q DF DtP V = = = C ϵ0 ϵr A/t ϵ0 ϵr DAE∆t V = ϵ0 ϵr A D ▶ g= is voltage sensitivity of crystal, a constant. ϵ0 ϵr V E electric field V = gtP ⇒ g = = = tP P stress 28 / 57 Desired Properties and Uses of Piezo-electric Materials ▶ Stability, high output, and insensitivity to temperature and humidity changes are desired. ▶ Quartz is one of the most stable ones, but its output is low. ▶ Rochelle salt provides highest output, but it is sensitive to humidity and temperatures exceeding 45◦ C. ▶ They are used for dynamic measurements, like acceleration and vibration measurements. ▶ The voltage developed under stress is not maintained in static conditions. ▶ Used in ultrasonic sensors. 29 / 57 Synchro Transmitter & Receiver ▶ The structure of synchro transmitter is similar to alternator ▶ Laminated steel stator hold Y connected, balanced 3-ϕ coils ▶ Rotor is dumb-bell shaped with salient pole construction ▶ The rotor of receiver is cylindrical shaped ▶ An a.c voltage V = Vm sin(ωc t) is applied to rotor 30 / 57  ▶ By transformer action, the voltages produced in stator coils w.r.t neutral are   2π Vs1n = Vm sin(ωc t) cos θR + 3 Vs1n = Vm sin(ωc t) cos (θR )   4π Vs1n = Vm sin(ωc t) cos θR + 3 Terminal voltages of stator are √   4π Vs1s2 = Vs1n − Vs2n = 3Vm sin(ωc t) sin θR + 3 √   2π Vs2s3 = Vs2n − Vs3n = 3Vm sin(ωc t) sin θR + 3 √ Vs3s1 = Vs3n − Vs1n = 3Vm sin(ωc t) sin (θR ) ▶ When θR = 0, Vs1n is maximum and Vs3s1 = 0 ▶ It is called Electrical zero position 31 / 57 ▶ The error signal generated at control transformer is e(t) = kVm cos(ϕ) sin(ωc t) π ϕ = − (θR − θc ) 2 ⇒ e(t) = kVm sin(ϕ) sin(ωc t) ▶ For small angular differences, sin(ϕ) ≈ ϕ e(t) = kVm (θR − θc ) sin(ωc t) ▶ Output at receiver is a suppressed-carrier modulated wave ▶ The peak values of error signal follows, em (t) = Ks (θR − θc ) ▶ Ks is sensitivity of error detector. ▶ It can be fed to servomotor mechanism to make it closed loop control system for position control ▶ Using a common a.c line for both the rotors results in Torque transmitter ▶ Rotation in transmitter causes imbalance of torque on receiver side, results in potential difference between two rotor coils. 32 / 57 ▶ Balance is restored by rotation in receiver side rotor 33 / 57 Resolvers ▶ Used to convert angular position into cartesian coordinates 34 / 57 ▶ When winding S1 S3 is excited and S2 S4 is short circuited ER1−3 = ksr ES1−3 cos(θ) ER2−4 = −ksr ES1−3 sin(θ) ▶ ksr is a coupling factor, assumed to be equal for any stator-rotor pair. ▶ When both the stator winding are excited ER1−3 = ksr (ES1−3 cos(θ) + ES2−4 sin(θ)) ER2−4 = ksr (ES2−4 cos(θ) − ES1−3 sin(θ)) ▶ Conversely, when the rotor windings are excited ES1−3 = krs (ER1−3 cos(θ) + ER2−4 sin(θ) ES2−4 = krs (ES2−4 cos(θ) − ES1−3 sin(θ)) ▶ Resolver can be classified into 1. Computing resolver: To generate trigonometric functions 2. Synchro resolver: For accurate data transmission 35 / 57 Fiber Optic Cables Cladding (n2 ) θ2 n1 > n2 Air (na ≈ 1) θ1 Core (n1 ) θ̄1 θa 36 / 57 Fiber Optic Cables ▶ By Snell’s Law, n1 sin(θ1 ) = n2 sin(θ2 )   n2 ▶ At critical angle: θ1 = θc and θ2 = 90◦ ⇒ θc = arcsin n1 ▶ For TIR, θ1 > θc ▶ Three different combinations of materials are used to construct OFCs: 1. Glass core and glass cladding 2. Glass core and plastic cladding 3. Plastic core and plastic cladding ▶ Based on refractive indexes, OFCs can be classified into: 1. Step-index OFC 2. Graded-index OFC 3. Single mode OFC ▶ Numerical aperture(NA) defines the cone of incidence q 1 × sin(θa ) = n1 × sin(θ̄c ) = n1 cos(θc ) = n1 1 − sin2 (θc ) s  2 q n2 = n1 1 − = n21 − n22 n1 37 / 57  2πr q ▶ The V parameter is given by V = n21 − n22 ; r is λ radius of core and λ is wavelength. ▶ Path of the light inside the core is a function of its angle of incidence ▶ The paths are called modes, number of modes for 4V 2 step-index OFC is given by, N ≈ 2 π ▶ Optical fibers used in sensors are mostly single mode ▶ For single mode fibers, V < 2.405 ▶ Radius of core for single mode fibers is very small, 5 to 10µm ▶ Pulse stretching occurs in multi-mode transmission due to different times of travel by different modes. ▶ Graded-index fibers have minimum pulse stretching ▶ The refractive index is gradually reduced from center to circumference of core ▶ Path taken by the light ray is parabolic, velocity towards the circumference is greater. 38 / 57 ▶ Acceptance angle (θ) is half the angle of cone of incidence, NA = sin(θ). At θ2 = θc , θa = θ ▶ Typical numerical aperture values for multi mode fibers: 0.2 to 0.6 and for single mode fibers: 0.03 to 0.1. ▶ Factors affecting the propagation of Light through optical sensors are: 1. Coherence of light 2. Size of fiber 3. Composition of fiber 4. Numerical aperture of source and fiber 5. Amount of light injected into fiber ▶ Fiber optic sensors can be classified into: 1. Pure fiber sensor: depend on environmentally induced changes in light travelling through fiber 1.1 Light can leak from core to cladding where it is absorbed 1.2 Light can be forced to travel a different path and compared with light travelling an alternative path to obtain phase difference. Can be observed by superposition. 2. Remote optic sensor: use optical fibers to carry light to a separate device that responds to light stimuli 39 / 57 ▶ A typical fiber optic sensor has a laser, sensing mechanism and detector. ▶ If the light signal changes due to some change in fiber, it is called intensity or intrinsic sensor. ▶ If the light the light signal changes due to some transducer type device, it is called interferometric or extrinsic sensor. ▶ Advantages of Fiber Optic Sensors: 1. Compatible with communications systems. 2. Don’t conduct electricity, useful in explosive environment and high voltage equipment. 3. Immune to electromagnetic interference. 4. Useful in measuring pressure, temperature, sound, displacement, and level. Can also be used in measuring electric current, voltage, electric and magnetic fields. 40 / 57 Photo Optic / Fiber Optic sensors ▶ Temperature Transducer: Refractive indexes of the fiber change with temperature. ▶ This results in change in the critical angle of fiber, which results in change in the leakage of light into cladding. ▶ Measurement of Sound: Can be performed by intrinsic or extrinsic type transducer ▶ Sound level is a function of phase shift 41 / 57 ▶ Reference cable is kept in stable environment ▶ When two beams are recombined, we get interference pattern 42 / 57 ▶ Measurement of Level: can be performed by submerging a carefully polished prism shaped end into a liquid of higher refractive index. ▶ TIR won’t happen when submerged. 43 / 57 Hall Effect Sensor ▶ Hall Effect: A current carrying strip in transverse magnetic field produces an emf across two edges as shown below, 44 / 57 ▶ Magnitude of the emf depends upon the current, flux density and Hall Effect Coefficient of material. K IB ▶ EH = H t KH is Hall effect coefficient; t is thickness of strip ▶ Hall effect emf is very small in conductors and difficult to measure. ▶ It is sufficiently large in some semiconductors like germanium. ▶ Magnetic to Electric Transducer: Used to measure magnetic field strength by inserting the semiconductor plate perpendicular to filed direction. ▶ It requires very small space, output is proportional to field density. ▶ Disadvantages are, it is sensitive to temperature and hall effect coefficient varies from plate to plate. 45 / 57 ▶ Measurement of Displacement: In the setup shown below, field strength at hall effect plate changes as a function of displacement by ferromagnetic plate. ▶ It can be used to measure displacements as small as 0.025mm 46 / 57 ▶ Measurement of Current: When a d.c. or a.c. is passed to through conductor, a magnetic field is generated at hall effect element ▶ It can measure current from less than a mA to thousands of amperes. ▶ Magnetic concentrator can be omitted at high currents ▶ Measurement of Power: A similar structure can be used to measure current through the current coil and calibrating the hall effect voltmeter in terms of power. 47 / 57 Photovoltaic Cells ▶ The photoelectric effect was first noted by a French physicist, Edmund Bequerel, in 1839. ▶ In 1905, Albert Einstein described the nature of light and the photoelectric effect. ▶ The first photovoltaic module was built by Bell Laboratories in 1954. ▶ Semiconductors are used in their construction. ▶ Due to its applications in space industry, the technology advanced, its reliability was established, and the cost began to decline. 48 / 57 Photovoltaic Transducers ▶ Output of photovoltaic cell is logarithmic in nature. ▶ The output is amplified with the help of op-amps or transistor circuits. ▶ Can be used in punched card reader to read hole patterns. ▶ PV sensor with op-amp in comparator mode can be used. ▶ Multiple sensors can be used to measure width of a material. 49 / 57 Digital Transducers ▶ They are capable of giving discrete information or pulses, compatible with digital computers. ▶ Used to get displacement of a tape, number of rotations of a shaft or revolutions of a point on disc. ▶ Can be classified into three types: 1. Tachometer transducers: ▶ They have a single output signal and can measure only unidirectional motion. ▶ Any motion in opposite direction could produce errors. ▶ Generally used to measure velocity. 2. Incremental transducers ▶ They have multiple output signals (two or three). ▶ They can also give the information of direction. ▶ If two tracks are used, one will be lagging 14 cycle. ▶ If a third one is also used, it gives a reference pulse per rotation. ▶ Widely used as quadrature encoders. ▶ Power supply to the encoder should be continuous, else readings will be erroneous. 50 / 57 ▶ 3 Absolute Transducers ▶ Generally used to measure single rotation. ▶ Output is a binary coded one with bits represented by pulses. ▶ Output is directly proportional to the position, discontinuous power will not affect the output. ▶ They are used in applications where the device is inactive for long durations and slow moving. ▶ Encoders can be constructed using contacting type and non-contacting type mechanisms. ▶ Contacting type encoders use brushes to close and open circuits. ▶ Non-contacting type encoders use a sheet of opaque and non-opaque sections between light source and detector pair. ▶ Can be constructed for measuring linear and angular displacements. 51 / 57 52 / 57 ▶ A conducting strip encircles the adjacent to MSB track or outer ring. ▶ Shaded areas are conducting and non-shaded areas with insulating materials. ▶ Circuits can be closed or opened with sliding contact made of brushes on each track. ▶ In binary code, there can be multiple bit transitions at a time. Eg. 7(0111) to 8(1000). ▶ Errors may occur in binary coded encoders due to small misalignments. ▶ Error in single bit can be large in decimal value. ▶ Corresponding gray code representation has only single bit transitions. Eg. 7(0100) to 8(1100). ▶ Gray code is less error prone compared to binary code. 53 / 57 ▶ Advantages of these encoders are 1. Relatively inexpensive 2. Can reach desired accuracy provided that there is sufficient space for tracks 3. Adequate for slow moving systems ▶ They suffer from wear and tear ▶ Resolution depends on number of bits 54 / 57 Decimal Binary Gray 0 0000 0000 1 0001 0001 2 0010 0011 3 0011 0010 4 0100 0110 5 0101 0111 6 0110 0101 7 0111 0100 8 1000 1100 9 1001 1101 10 1010 1111 11 1011 1110 12 1100 1010 13 1101 1011 14 1110 1001 15 1111 1000 55 / 57 Shaft Encoders ▶ Shaft encoders has a resolution of 360◦ 2n. ▶ Coding errors can be minimized by using gray codes and double-brush systems (V or U pattern). 56 / 57 ▶ When LSB brush reads 0, next leading brush is read, otherwise next lagging brush is read. ▶ Optical encoders: ▶ They are non-contacting type encoders, tracks with opaque and transparent section are used. ▶ A light source is used on one side and photo detectors on the other side. ▶ Moire’s fringe techniques can also be used for higher accuracy. ▶ Optical encoders can also be divided into three types: 1. Tachometer encoders 2. Incremental encoders 3. Absolute encoders ▶ Optical sensors generally have higher accuracy compared to brush type encoders. ▶ When coupled with gray code, they are very accurate. 57 / 57